1. Field of the Invention
The present invention relates to a two-shaft fluid machine, for example, used for a fun and a compressor, specifically for a supercharger of a vehicle.
2. Description of the Related Art
A Roots displacement mechanical supercharger 201 for the conventional two-shaft fluid machine as shown in FIG. 1 is described in Japanese Patent Application Laid-Open No. 6-12987.
The Roots displacement mechanical supercharger 201 is constituted by a pair of rotors 203 and 205 meshed with each other in a tooth trace in parallel to a rotating central axis, a casing 209 for rotatably receiving them in a rotor chamber 207, a set of timing gears 211 synchronously rotating each of the rotors 203 and 205 and making them meshed in such a manner as not to be in contact with each other, an input pulley 213 and the like.
The input pulley 213 is fixed to a rotor shaft 215 of the input end rotor 203, and is connected to the engine end pulley through a belt. When the rotors 203 and 205 are rotated, an intake air is sucked to the rotor chamber 207 from a fluid inlet port of the casing 209, and is discharged from a fluid outlet port so as to be fed to the suction end of the engine.
In the Roots displacement mechanical supercharger 201, in order to prevent each of the rotors 203 and 205 from being brought into contact with each other, it is necessary to provide a suitable gap between them, and further it is desirable to make the gap as narrow as possible so as to miniaturize an air leakage.
Then, in a conventional art, a thick coating having a general thickness of about 0.4 mm is given on all the surface of each of the rotors 203 and 205, and each of the rotors 203 and 205 is rotated and fitted, thereby cutting down the contact coating with each other, so that a treatment for forming a suitable gap is performed.
However, since the coating of the rotor is expensive, it is desirable to reduce the cost by making the coating thin, however, when the coating is made thin, there occurs an exposure of a surface of the rotor due to a slight contact, and there is a fear that a seizure and sticking is generated.
A certain degree of gap is provided between tips of the respective rotors 203 and 205, and between the tips of the respective rotors 203 and 205 and the rotor chamber 207 in order to prevent contact as mentioned above, however, since the pga between the tips of the rotors 203 and 205 and the rotor chamber 207 is set to be narrow, a problem of peeling of the coating easily occurs at an end surface of the rotors 203 and 205.
Further, a tensile force of the belt applied to the input end rotor 203 through the input pulley 213 is given to the bearing 217 and 219 supporting the rotor shaft 215, the rotor 203 is inclined by a fine gap disposed between the internal and external races of the bearing and the rolling element, and a thermal expansion of the casing 209 is added, so that the end surface of the rotor 203 and the rotor chamber 207 are brought into contact with each other and the coating is easily peeled.
Still further, in the Roots displacement mechanical supercharger 201 described above, since the length (radius of the rotor) between the central axis of the rotor and the outer peripheral portion of the rotor is grate relating to the other length between the bearings supporting the rotor, the contact between the rotor end portion of the rotor and the opposing surface of the rotor chamber can be occurred when the rotor is inclined.